Evaluation of the antioxidant capacity of ubiquinol and dihydrolipoic acid.

Ubiquinone and alpha-lipoic acid are natural constituents which are involved in mitochondrial energy metabolism. Their bioenergetic activities require redox-cycling. In the case of alpha-lipoic acid redox-cycling leads to dihydrolipoic acid which occurs in multienzyme complexes involved in the citric acid cycle while UQ recycles through semi- and divalently reduced ubiquinones in the respiratory chain. We have proved the validity of the concept about the antioxidant function of these natural compounds in their reduced form. Ubiquinol was found to interfere with lipid peroxidation of liposomal membranes being itself degradated by two consecutive oxidation steps. Dihydrolipoic acid was found to totally recycle ubiquinone to the antioxidant active divalently reduced form. In contrast to the antioxidative derived reaction products of ubiquinols which in turn promoted lipid peroxidation, the antioxidant derived reaction product of dihydrolipoic acid was the unreactive two electron oxidation product alpha-lipoic acid. Our experiments demonstrate the existence of an dihydrolipoic acid driven recycling of UQ to the antioxidative-active UQH2. The efficiency of the antioxidative capacity of the latter was found to be diminished through prooxidant activities of the antioxidant-derived metabolites.